Acceleration sensor

ABSTRACT

An acceleration sensor has an oscillating structure which is movably suspended on a substrate and can be deflected by an acting acceleration. The acceleration sensor also has an analyzing arrangement detecting a deflection of the oscillating structure due to the acceleration. The oscillating structure and/or the analyzing arrangement are connected to the substrate by mechanical decoupling devices.

BACKGROUND INFORMATION

Conventional acceleration sensors generally have an oscillatingstructure as a seismic mass suspended movably on a substrate. Thisseismic mass is deflected by an acceleration acting on it and changesits position relative to the substrate. Analyzing means are provided forthe seismic mass to detect the degree of deflection due to acceleration.Known analyzing means include piezoresistive, capacitive andfrequency-analog analyzing arrangements, for example. With capacitiveanalyzing means, the seismic mass is provided with a comb structure thatworks together with a stationary (i.e., attached to the substrate) combstructure. Capacitances that vary in size with deflection of the seismicmass develop between the individual webs of the comb structures. Thesechanges in capacitance can be detected by analyzing circuits, and thusan acceleration acting on the acceleration sensor can be detected.

One disadvantage of the known acceleration sensors is that there may befluctuations in the length of the substrate or the sensor structuresdepending on temperature or mechanical stresses, for example, causingminor variations in the positions of the seismic mass suspended on thesubstrate or in the analyzing means, subsequently causing a change inthe signal. These signal changes lead to faulty detection of an actingacceleration or they are superimposed on a signal of the analyzing meanswhich is proportional to an acting acceleration with an offset error.

SUMMARY OF THE INVENTION

The acceleration sensor according to the present invention isadvantageous in that fluctuations dependent on temperature or mechanicalstresses occurring in the substrate or sensor structures can becompensated. Due to the fact that the oscillating structure and/oranalyzing arrangement are connected to the substrate by mechanicaldecoupling devices, it is advantageously possible to compensate for anymaterial effects in the substrate or sensor structures caused bycompression and/or tensile stresses as well as fluctuations intemperature, so that they have no effect on the acceleration sensor, inparticular its sensitivity. In addition, differences in material betweenthe substrate and the sensor can be compensated with the mechanicaldecoupling devices, e.g. with acceleration sensors applied to a wafer byadditive methods of surface micromechanics. Thus, for example,differences in the thermal expansion properties of silicon and metallicmaterials such as those used in some additive techniques can becompensated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of a first embodiment of an acceleration sensoraccording to of the present invention.

FIG. 2 shows a top view of a second embodiment of the accelerationsensor according to the present invention.

FIG. 3 shows a top view of an analyzing arrangement of the accelerationsensor.

DETAILED DESCRIPTION

FIG. 1 shows a top view of the design of an acceleration sensor 10.Acceleration sensor 10 is structured on a substrate, such as a wafer(not shown in detail). The structuring can be accomplished by knownmethods of surface micromechanics. In FIG. 1, the wafer is formed by theplane of the paper. The wafer may at the same time have electricanalyzing circuits, not described in detail here, for accelerationsensor 10.

Acceleration sensor 10 has an oscillating structure 12 designed as aseismic mass 14. Oscillating structure 12 is suspended movably withrespect to the substrate (wafer). A frame 16 is provided for thispurpose, with projections 18 extending inward from it. Projections 18are connected to one another by spring bars 20, with seismic mass 14being arranged between opposing spring bars 20. As seen in a top view,spring bars 20 have a small width, and as seen into the plane of thepaper, they have a relatively great depth. Frame 16 is connected toholding webs 24 by decoupling webs 22. Decoupling webs 22 also have arelatively small width as seen in a top view and a relatively greatdepth as seen into the plane of the paper. Holding webs 24 are attachedto the substrate (wafer) by central anchoring points 26. Theserelatively small fastening points 26 support the entire arrangement ofholding webs 24, frame 16, oscillating structure 12 and spring webs 20or decoupling webs 22, and this arrangement being arranged over thesubstrate but otherwise floating freely. This can be accomplished byknown process steps in the production of surface micromechanicalstructures, with undercutting below the freely vibrating areas, so thatthere is a slight gap between the substrate and the arrangement.

Seismic mass 14 has a comb structure 28 formed by lamellae arrangedperpendicular to the surface of the wafer on both sides. Comb structures28 are relatively rigid, so that with any movement of seismic mass 14,they vibrate rigidly with seismic mass 14.

Acceleration sensor 10 also has analyzing means 30 (e.g., an analyzingarrangement) in the form of stationary comb structures 32. Combstructures 32 project from a holding bar 34 connected to a holding web38 by decoupling webs 36. Holding web 38 is connected by an anchoringpoint 40 to the substrate (wafer). Here again, only holding web 38 isconnected to the substrate in the area of anchoring points 40, so theother areas of holding web 30, decoupling webs 36, holding bars 34 andcomb structures 32 are cantilevered, i.e., they have no direct point ofcontact with the substrate.

Comb structures 28 and 32 of seismic mass 14 and/or analyzing means 30mesh with one another, forming a capacitive analyzing means in a knownway. Due to the arrangement of comb structures 32 and 28, capacitances Cdevelop between adjacent webs of comb structures 28 and 32, i.e., acapacitance C1 at the left and a capacitance C2 at the right in the topview shown in FIG. 1. These capacitances are determined by the distancebetween the webs of comb structures 28 and 32 and by the opposingsurfaces of the webs of comb structures 28 and 32. The entireacceleration sensor 10 is an electrically conducting material such assilicon, so the capacitances can be linked to the substrate and thus toan analyzing circuit (not shown in detail) by anchoring points 26 and40.

Acceleration sensor 10 has decoupling webs 22 and anchoring points 26and 40 that are arranged symmetrically with an imaginary center line 42passing through seismic mass 14. Comb structures 28 and 32 are arrangedin mirror image symmetrically to one another. Anchoring points 26 and 40all lie on one imaginary line 44. This line 44 is parallel to combstructures 28, 32. As a result, the distance between the individualelements remains the same when the comb structures are stretched, andtherefore the capacitance between the comb structures remainsessentially unchanged. This further reduces the dependence ontemperature, because the structure can expand or contract freely in thedirection perpendicular to this line 44, so that no thermal stresses canbe induced. Thermal stresses along this line are also relaxed inconjunction with the decoupling elements. Expansion of the materialperpendicular to this line takes place symmetrically with it, namelyidentically for the suspended mass with its movable comb structures andits comb structures fixed in stationary mount on this line, togetherforming the analyzing means of the sensor element. Since both“stationary” and “movable” comb structures can expand or relaxidentically perpendicular to this line, the gap spacings of theanalyzing means change only by ˜ΔTε_(sensor material), which isrelatively minor. Otherwise, in an extreme case the gap width wouldchange by ˜ΔT·(ε_(sensor material)−ε_(substrate))·I, in the case of acomb attached completely to the stationary mount (substrate), with Ibeing the lateral extent of the comb expansion.

Acceleration sensor 10 shown in FIG. 1 carries out the followingfunctions:

Seismic mass 14 is deflected according to an acceleration acting onacceleration sensor 10 as indicated by double arrow 46. Due to thedesign of spring bars 20, deflection is permitted only in the directionof the possible accelerations according to double arrow 46, because ofthe soft suspension of seismic mass 14 in this direction by spring bars20 and the rigid suspension in the direction perpendicular to that. Dueto the deflection of seismic mass 14, the distances between the webs ofcomb structures 28 and 32 change, resulting in a corresponding change incapacitances C1 and C2. When seismic mass 14 is deflected upward, thedistance between the webs of comb structures 28 and 32 on the left sideof the seismic mass is reduced, while the distance between the webs ofcomb structures 28 and 32 on the right side of seismic mass 14 isincreased accordingly. Similarly, capacitances C1 and C2, which can bedetected by a corresponding analyzing circuit and supply a signalcorresponding to acting acceleration 46 are decreased and increased,respectively. This signal can be used, for example, to deploy restraintsystems in motor vehicles.

The arrangement of decoupling webs 22 and the arrangement of anchoringpoints 26 and 40 on line 44 yield the result that stresses within thesubstrate which depend on the temperature and/or material and areindependent of the action of acceleration 46 are not transmitted toseismic mass 14 or to analyzing means 30 but instead can be relaxedfreely. The mechanical stresses and/or temperature-dependent expansionsoccurring in the substrate are compensated by decoupling webs 22, sothey cannot be transmitted to frame 16 and seismic masses 14 arranged init. Decoupling webs 22 are soft and flexible in the plane of thesubstrate, so that, for example, a change in the length of the materialwhich is transmitted to holding webs 24 can be compensated. This isachieved because decoupling webs 22 are relatively narrow as seen in theview from above, and they have a relatively great depth as seen into theplane. Therefore, decoupling webs 42 have a relatively soft suspensionin the direction of center line 42. Due to the arc-shaped geometry ofdecoupling webs 22 between frame 16 and holding webs 24, changes inlength occurring in the direction of line 44 can also be compensated.Due to the great depth relative to the width of decoupling webs 22,deflections perpendicular to the plane of the substrate (perpendicularto the plane of the paper in FIG. 1) are compensated, because decouplingwebs 22 are rigid in this direction. Due to the symmetrical arrangementof decoupling webs 22 and anchoring points 26 and 40, mechanicalstresses that occur are compensated uniformly and with the oppositesign, so that seismic mass 14 and analyzing means 30 remain in theirpositions.

A change in the length of the structure of acceleration sensor 10itself, for example, spring bars 20 and/or frame 16 and/or seismic mass14 with a change in temperature, where the change in length results fromthe coefficient of linear expansion of the respective material, iscompensated by decoupling webs 22 and the arrangement of anchoringpoints 26 and 40 in a line. This does not result in any change inposition of seismic mass 14 relative to analyzing means 30.

FIG. 2 shows another embodiment of an acceleration sensor 10. The sameparts as in FIG. 1 are labeled with the same reference notation and willnot be explained again. In the embodiment in FIG. 2, decoupling webs 22are designed as angles. This also achieves the result that decouplingwebs 22 are soft in the direction of center line 42 and in the directionof line 44, and they are stiff in the direction into the plane of thepaper due to their relatively great depth in relation to their width asseen in a top view. Thus, changes in length occurring here in thesubstrate or in acceleration sensor 10 due to changes in temperature ormechanical stresses can also be compensated. The angular design ofdecoupling webs 22 can be achieved more easily with known methods ofstructuring structures by surface micromechanics than the curvedstructure of decoupling webs 22 shown in FIG. 1. The effect ofdecoupling webs 22 is the same in both cases.

FIG. 3 shows a detailed view of one exemplary embodiment of the designof analyzing means 30. Holding bar 34 is attached to holding webs 38 bya frame 46 and decoupling webs 48. Holding webs 38 in turn are attachedto the substrate by anchoring points 40.

In addition, decoupling of analyzing means 30 from changes in length ofthe substrate or of the analyzing means 30 itself is also achieved withthe embodiment illustrated in FIG. 3. In FIG. 3, improved decoupling ofanalyzing means 30 from changes in length due to temperature andmechanical stresses in the material of the substrate is achieved throughthe symmetrical arrangement of decoupling webs 48 and the arrangement ofanchoring points 40 on line 44. Decoupling webs 48 are in turn designedwith an angular shape, so they are soft in the direction of line 44 andcenter line 42 (see FIG. 1) and they are rigid perpendicular to thesubstrate.

On the whole, complete compensation of temperature and stresses ispossible with decoupling webs 22 and 48 and the arrangement of anchoringpoints 26 and 40 on one line, so that any compressive and tensilestresses in the sensor material, i.e., in the substrate or inacceleration sensor 10 itself, can be compensated and thus have noeffect on the sensor response, in particular an offset and asensitivity.

What is claimed is:
 1. An acceleration sensor, comprising: anoscillating structure which is arranged on a substrate in a movablysuspended manner, the oscillating structure capable of being deflectedby an acting acceleration; at least one mechanical decoupling device; ananalyzing arrangement detecting a deflection of the oscillatingstructure due to the acting acceleration, wherein at least one of theoscillating structure and the analyzing arrangement is connected to thesubstrate by the at least one mechanical decoupling device; decouplingwebs; and holding webs attached to the substrate; wherein: the at leastone mechanical decoupling device includes a frame connected to theholding webs via the decoupling webs; and the oscillating structure isarranged in a suspension mount in the frame.
 2. The acceleration sensoraccording to claim 1, wherein the analyzing arrangement is connected bythe decoupling webs to at least one of the holding webs.
 3. Theacceleration sensor according to claim 2, wherein the decoupling webshave an angled shape between the analyzing arrangement and the at leastone of the holding webs.
 4. The acceleration sensor according to claim2, wherein each of the at least one of the holding webs is connected tothe substrate via a symmetrically-arranged anchoring point.
 5. Theacceleration sensor according to claim 2, wherein the decoupling websare soft in a first direction of motion which is parallel to a surfaceof the substrate, and wherein the decoupling webs are rigid in a seconddirection of motion which is perpendicular to the surface of thesubstrate.
 6. The acceleration sensor according to claim 4, wherein theanchoring point is located on an imaginary line extending along thesubstrate.
 7. The acceleration sensor according to claim 6, furthercomprising: comb structures composed of elongated plate-shaped webs, thecomb structures being arranged parallel to one another, wherein theimaginary line extends parallel to the elongated plate-shaped webs. 8.The acceleration sensor according to claim 1, wherein each of theholding webs is connected to the substrate at a symmetrically-arrangedanchoring point.
 9. The acceleration sensor according to claim 8,wherein the anchoring point is located on an imaginary line extendingalong the substrate.
 10. The acceleration sensor according to claim 9,further comprising: comb structures composed of elongated plate-shapedwebs, the comb structures being arranged parallel to one another,wherein the imaginary line extends parallel to the elongatedplate-shaped webs.
 11. The acceleration sensor according to claim 1,wherein the decoupling webs have an arc shape between the frame and theholding webs.
 12. The acceleration sensor according to claim 1, whereinthe decoupling webs have an angled shape between the frame and theholding webs.
 13. The acceleration sensor according to claim 1, whereinthe decoupling webs include four symmetrically-arranged decoupling webs,and wherein the frame is connected to the holding webs by the fourdecoupling webs.
 14. The acceleration sensor according to claim 1,wherein the oscillating structure is connected to the substrate by theat least one mechanical decoupling device.
 15. The acceleration sensoraccording to claim 1, wherein the analyzing arrangement is connected tothe substrate by the at least one mechanical decoupling device.
 16. Anacceleration sensor, comprising: an oscillating structure which isarranged on a substrate in a movably suspended manner, the oscillatingstructure capable of being deflected by an acting acceleration; at leastone mechanical decoupling device; an analyzing arrangement detecting adeflection of the oscillating structure due to the acting acceleration,wherein the oscillating structure and the analyzing arrangement areconnected to the substrate by the at least one mechanical decouplingdevice; decoupling webs; and holding webs attached to the substrate,wherein the at least one mechanical decoupling device includes a framewhich is connected to the holding webs via the decoupling webs, andwherein the oscillating structure is arranged in a suspension mount inthe frame.
 17. The acceleration sensor according to claim 16, whereineach of the holding webs is connected to the substrate at asymmetrically-arranged anchoring point.
 18. The acceleration sensoraccording to claim 17, wherein the anchoring point is located on animaginary line extending along the substrate.
 19. The accelerationsensor according to claim 18, further comprising: comb structurescomposed of elongated plate-shaped webs, the comb structures beingarranged parallel to one another, wherein the imaginary line extendsparallel to the elongated plate-shaped webs.
 20. The acceleration sensoraccording to claim 16, wherein the decoupling webs have an arc shapebetween the frame and the holding webs.
 21. The acceleration sensoraccording to claim 16, wherein the decoupling webs have an angled shapebetween the frame and the holding webs.
 22. The acceleration sensoraccording to claim 16, wherein the decoupling webs include foursymmetrically-arranged decoupling webs, and wherein the frame isconnected to the holding webs by the four decoupling webs.
 23. Anacceleration sensor, comprising: an oscillating structure which isarranged on a substrate in a movably suspended manner, the oscillatingstructure capable of being deflected by an acting acceleration; at leastone mechanical decoupling device; an analyzing arrangement detecting adeflection of the oscillating structure due to the acting acceleration,wherein the oscillating structure and the analyzing arrangement areconnected to the substrate by the at least one mechanical decouplingdevice; decoupling webs; and at least one holding web attached to thesubstrate, wherein the analyzing arrangement is connected by thedecoupling webs to the at least one holding web; wherein the at leastone mechanical decoupling device includes a frame which is connected tothe at least one holding web via the decoupling webs, and wherein theanalyzing arrangement is connected to the frame.
 24. The accelerationsensor according to claim 23, wherein the decoupling webs include foursymmetrically arranged decoupling webs, and wherein the frame isconnected to the at least one holding web by the four symmetricallyarranged decoupling webs.